1. Field of the Disclosure
The present disclosure relates to an electrostatic capacitance detection circuit that detects a minute change in electrostatic capacitance on a sensor such as a touch pad and a touch sensor, and an input device.
2. Description of the Related Art
In the related art, there has been proposed an electrostatic capacitance detection circuit that is suitable for detecting a minute change in electrostatic capacitance between sensor electrodes in an input device such as a touch pad and a touch sensor in a noisy situation.
For example, the electrostatic capacitance detection circuit disclosed in Japanese Patent No. 4275865 employs a configuration in which, to detect mutual capacitance between sensor electrodes, electric charge transfer from the mutual capacitance is performed with respect to an integral capacitor at the timing for when a rising edge of a drive pulse occurs. Furthermore, in the electrostatic capacitance detection circuit disclosed in U.S. Unexamined Patent Application Publication No. 2011-0273400, a filtering effect on the noise with a low frequency is improved by performing the electric charge transfer from the mutual capacitance to two integral circuits at the timing for when both edges of the drive pulse occur.
Incidentally, inter-electrode capacitance (the mutual capacitance) between a drive electrode and a detection electrode that makes up a sensor is normally as small as several pF, but an amount of change due to finger's close proximity is smaller than this. That is, it is on the order of several hundred fF or less. For this reason, an influence of mixture noise is serious. Causes of the mixture noise, can be a noise from a power source of a system into which the touch pad and the touch sensor are built, a drive signal of a liquid panel inside the system and others, and an influence of these noise sources cannot be ignored due to the complexity of the apparatus and others.
In a case of the detection circuit disclosed in Japanese Patent No. 4275865, the electric charge transfer to the integral capacitor is performed only on the rising edge of the drive pulse. Because of this, when a noise is applied to a manipulation body, such as a finger, or the noise is applied to the system that detects the electrostatic capacitance, the noise is mixed into the electric charge that is transferred. When a frequency of the noise applied during an integral period is lowered, the averaging of the mixture noise is not sufficient during the integral period, and a larger influence of the noise on output data appears.
In the case of the detect circuit disclosed in U.S. Unexamined Patent Application Publication No. 2011-0273400, the electric charge transfer is performed on both of the edges of the drive pulse and thus the filtering effect on the noise with the low frequency is improved, but an integral circuit, using an operational amplifier, is necessary for two systems, and circuit scale and power consumption are caused to be increased.